This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Ferrous and ferric ions play key roles in many biological systems for the catalytic generation of free radicals through Fenton chemistry. High field ESR, due to its superior orientational resolution and sensitivity to ions that are otherwise 'ESR silent'at lower frequencies, can offer insights into the ligand structure of ferric and ferrous ions and thus inform the elucidation of function of these systems. In order to gain the necessary expertise for studying these heterogeneous systems, we have been studying model compounds, in single crystals, that contain ferrous and ferric ions. The linewidths in ferrous and ferric systems are, generally speaking, quite broad and only readily observable at liquid helium temperatures. Nevertheless, we have obtained well-resolved single crystal spectra on both ferrous and ferric systems and we are developing the technology for studying systems of more direct biological significance.